Citric acid is produced commercially by fermentation of carbohydrates. By one known method, essentially clean carbohydrates such as glucose or refined sucrose are used for the fermentation and this yields a relatively clean fermentation broth from which citric acid can be recovered by direct extraction with a water-immiscible organic extractant followed by water stripping of the resulting extract as described, for example, in U.S. Pat. Nos. 3,944,606, 4,275,234 and 4,334,095. According to U.S. Pat. No. 3,944,606 alkali metal or ammonium citrates are produced directly from a fermentation broth by extraction with a water-immiscible organic extractant consisting of a mixture of a secondary or tertiary amine with an organic solvent, and the resulting organic extract is re-extracted with a compound that forms an alkali metal or ammonium salt of citric acid.
According to U.S. Pat. No. 4,275,234 a citric acid fermentation broth is extracted with a water-immiscible organic extractant which comprises a solution of at least one secondary or tertiary amine in which the aggregate number of carbon atoms is at least 20 in admixture with hydrocarbon and alcohol and the resulting extract is stripped with water at a temperature which is higher, preferably by at least 20.degree. C. than the broth extraction temperature.
According to U.S. Pat. No. 4,344,095 a citric acid fermentation broth is extracted with a mixture of a water-immiscible amine and a water-immiscible organic acid dissolved in a suitable water-immiscible solvent, and the resulting extract is back-extracted with water.
The most common source of carbohydrate for citric acid fermentation is beet molasses. Citric acid fermentation on such a nutrient yields a fermentation broth rich in cations and the yield of direct extraction thereof is poor. Accordingly, the recovery of citric acid from such broth is usually by way of the so-called "lime/sulfuric acid process". This process comprises a so-called "liming" operation, i.e. treatment of the fermentation broth with calcium hydroxide to yield calcium citrate which is filtered off, washed and decomposed with aqueous sulfuric acid. The calcium sulfate that forms is filtered off and the resulting aqueous acidic solution is gradually evaporated in a crystallizer whereupon citric acid crystallizes leaving behind a saturated mother liquor which holds significant residual amounts of citric acid together with accumulated impurities. This mother liquor will be referred to hereinafter as "lime sulfuric acid/citric acid mother liquor".
From the lime sulfuric acid/citric acid mother liquor the residual amounts of citric acid cannot be recovered by crystallization because this would inevitably lead to the inclusion of impurities in the crystallizing citric acid. Accordingly, residual amounts of citric acid are recovered from a lime sulfuric acid/citric acid mother liquor by multi-stage counter-current water extraction with a water-immiscible extractant, for example in the manner disclosed in EP 0432610 using as extractant a mixture of at least one organic amine and a liquid hydrocarbon.
From the extraction of a sulfuric acid/citric acid mother liquor with an organic extractant, there results an organic extract whose degree of purity is similar to that of an extract obtained by direct extraction of a fermentation broth, e.g. in accordance with the teachings of any of U.S. Pat. Nos. 3,944,606, 4,275,234 and 4,334,095. It would therefore be of advantage if such extracts could be combined for the purpose of the recovery of citric acid values therefrom either in the form of free acid or as alkali metal or ammonium citrates. However, simple combination of such organic citric acid extracts is impractical because of the different concentrations thereof, the extract resulting from the multi-stage counter-current extraction of a lime sulfuric acid/citric acid mother liquor being as a rule more concentrated than the extract obtained from direct organic solvent extraction of a fermentation broth. Accordingly, a mere combination of the final extracts would result in dilution which is disadvantageous for the recovery of citric acid values.
It is the object of the present invention to provide a process by which a first organic extraction citric acid extract obtained by multi-stage counter-current extraction of a lime sulfuric acid/citric acid mother liquor and a second organic extractant citric acid extract obtained by direct extraction of a fermentation broth can be combined to yield as product an organic extractant solution of citric acid of the same concentration as that which would be obtained by extraction of the lime sulfuric acid/citric acid mother liquor without the addition of any second organic citric acid extract.